Objective:S.W.A.B.T. • Explain how an air mass forms.

• List the four main types of air masses.

• Describe how air masses affect the weather of North America.

• Compare the characteristic weather patterns of cold fronts with those of warm fronts

• Identify from diagram the various types of fronts and the symbols used in meteorology

Activities:

• Warm-ups

• Notes/discussion

• Diagram labeling

• Worksheet North American Air Masses 1-4 and

Warm and Cold fronts 1-4

Weather 21-1 Air Masses & 21.2 Fronts

Formation of Air Masses

• Differences in air pressure are caused by unequal heating of Earth’s surface. Hot air rises – cool air sinks

Air Masses

• Differences in air pressure at different locations on Earth create wind patterns.

How Air Moves • Air moves from areas of high pressure to areas of

low pressure. Therefore, there is a general, worldwide movement of surface air from the poles toward the equator.

Formation of Air Masses • Air Mass a large body of air throughout which

temperature and moisture content are similar.

1. - They take on properties where they formed

2. - Air masses that form over frozen polar regions are very cold and dry. Air masses that form over tropical oceans are warm and moist.

Types of Air Masses Air masses are classified according to their source regions.

The source regions for cold air masses are polar areas.

The source regions for warm air masses are tropical areas.

Air masses that form over the ocean are called maritime. Air masses that form over land are called continental.

The combination of tropical or polar air and continental or maritime air results in air masses that have distinct characteristics.

Continental Air Masses • There are two types of continental air masses:

continental polar (cP) and continental tropical (cT).

• Continental polar air masses are cold and dry. Continental tropical air masses are warm and dry.

Maritime Air Masses The two types of maritime air masses are maritime polar (mP) and maritime tropical (mT).

• Maritime polar air masses are moist and cold.

• Maritime tropical air masses are moist and warm.

• When these very moist masses of air travel to a new location, they commonly bring precipitation and fog.

North American Air Masses • The diagram below shows the four main types of air

masses that influence North America.

Fronts A front is when two unlike air masses meet - it is a boundary area.

Fronts A cool air mass is dense and does not mix well with the

less-dense air of a warm air mass, so a boundary, called a front, forms between air masses.

Changes in weather usually take place along the various

types of fronts.

Fronts A cold front is the front edge of a moving mass of cold

air that pushes beneath a warmer air mass like a wedge

Cold Cold front symbol:

Fronts Cold fronts bring a long line of heavy thunderstorms, called a squall line, may occur in the warm, moist air just ahead of a fast-moving cold front.

A slow-moving cold front

typically produces weaker storms and lighter

precipitation than a fast-moving cold front does.

Fronts A warm front is the front edge of advancing warm air

mass that replaces colder air with warmer air Warm front symbol:

Fronts The slope of a warm front is gradual. Because of this

gentle slope, clouds may extend far ahead of the base of the front.

A warm front generally produces slow steady precipitation

over a large area.

Fronts Sometimes, when air masses meet, the cold moves

parallel to the front, and neither air mass is displaced. A stationary front is a front

of air masses that moves either very slowly or not at all

Produces steady precipitation near front as long as the front does not move Symbol:

Fronts Occluded front – occurs when a cold front overtakes a warm front, due to cold fronts move faster

- Produces steady precipitation

- Warm air is pushed upward rapidly

- Symbol: